This invention relates to methods of manufacturing integral structural members from materials such as metal alloys utilizing superplastic forming and diffusion bonding techniques.
Certain metal alloys such as, for example, titanium alloys are known to exhibit superplastic forming and diffusion bonding properties within specific temperature and pressure ranges; and integral structural members can advantageously be manufactured from such alloys using superplastic forming and diffusion bonding techniques.
One serious problem encountered in the manufacture of structural members by a process of superplastic forming and diffusion bonding is that where two workpieces are joined to form an integral structural member having two generally planar members orientated transversely to each other the transition section between the two pieces will include a fillet along only one side thereof. In such cases the transition section will be subject to stress concentration and will present a weakness in the structural member. It is therefore essential to provide, in such integral structural members, a transition section having a fillet along both sides thereof; and the feasibility of utilizing superplastic forming and diffusion bonding techniques in manufacturing such members depends on a workable process for providing a fillet along the second or opposite side of the transition section.
The U.S. Pat. No. 3,340,101 to Fields, Jr. et al, which issued Sept. 5, 1967, discloses a process for forming certain metals using superplastic forming techniques.
The U.S. Pat. No. 3,920,175 to Hamilton et al, which issued Nov. 18, 1975, discloses a method of fabricating metal structures from a blank and other workpieces by superplastically forming the blank and concurrently diffusion bonding it to the workpieces.
The U.S. Pat. No. 4,084,029 to Johnson et al, issued Apr. 11, 1978, and assigned to the same assignee as the present application discloses a sine-wave beam construction wherein a filler material of plastic or the like is added to a pocket formed by adjacent flange portions and a capstrip. The filler material is added to provide additional beam strength and stiffness. The disclosed beam is constructed from composite materials, and neither superplastic forming nor diffusion bonding are part of the beam's manufacture.
The U.S. Pat. No. 3,950,841 to Conn, which issued Apr. 20, 1976, discloses a method which includes diffusion bonding of metalic members and wherein homogeneous filleted junctures between the members are provided. As disclosed, this method requires ramming tools and a plurality of restraining tools in order to accomplish a joining of the members and creation of the filleted junctures.
Unlike any of the processes disclosed in the foregoing patents, the present invention provides an integral structural member comprising two metal pieces superplastically formed and diffusion bonded together in a single operation with the transition section between the pieces including an integral fillet portion along each side of the transition section.